Many automobiles are equipped with a transmission that generally includes such a ratio change mechanism, whose operation is controlled by applying a predetermined pressure of hydraulic oil (line pressure). For example, Japanese Laid-Open Patent Publication No. S60(1985)-256662 discloses a system that applies a hydraulic pressure into a hydraulic cylinder for controlling the width of a pulley, or more specifically, the force applied on the pulley pushing the belt in a belt-type continuously variable transmission. This control system operates to calculate the torque output from the engine on the basis of the rotational speed of the engine and of the pressure inside the intake pipe (negative pressure) and to set the hydraulic pressure in correspondence to the calculated torque value.
By setting the hydraulic pressure in correspondence to the torque output of the engine, it is possible that the engaging capacity of the clutch in the transmission, which is set also by the hydraulic pressure, be set to a minimum capacity required for transmitting the engine-output torque to the side toward the wheels. This can reduce the energy consumed by the engine for generating the hydraulic pressure and thereby improve the fuel efficiency of the vehicle. Moreover, this improves the operability and performance of the vehicle. Because the clutching capacity is limited to the minimum necessary for the transmission of the engine-output torque, the clutch can slip limiting the torque change while the output of the engine or the load of the driving vehicle is changing rapidly (for example, when the accelerator pedal is operated quickly or the vehicle goes over a curb). Furthermore, this attempt of making the pushing force applied to the belt minimum improves the durability of the belt and also facilitates miniaturization in designing the transmission.
In recent years, for the purpose of improving fuel economy, vehicles are equipped with an idling elimination control, in which the operation of the engine is stopped when the vehicle comes into a halt, or equipped with a partial cylinder operation control, in which, under certain driving conditions, some of the cylinders are deactivated or disengaged from the operation of the engine. The output of the engine is smaller while some of the cylinders are deactivated (this condition is referred to as “partial cylinder operation”) than while all the cylinders are active or engaged in the operation (this condition is referred to as “complete cylinder operation”). Because of this reason, for example, Japanese Laid-Open Patent Publication No. S59(1984)-13154 discloses a method for increasing the speed ratio of a ratio change mechanism used with such an engine, which has cylinders that are deactivated in partial cylinder operation mode.